1. Field of the Invention
The present invention relates to a display device, particularly an active matrix display device having self-light emitting elements, and to a driving method of a display device. The invention further relates to a semiconductor integrated circuit incorporated in a display device.
2. Description of the Related Art
In recent years, a display device using a light emitting element (self-light emitting element) has been studied and developed. Such a display device has the advantages of high image quality, reduced thickness, light weight or the like, and thus it is widely used as a screen of a mobile phone and a monitor of a personal computer. In particular, since the light emitting display device has the features such as low voltage drive, low power consumption, and fast response which contributes to dynamic display, it is expected to be used for various electronic apparatuses typified by a next-generation mobile phone and a portable information terminal (PDA).
A light emitting element is also called an organic light emitting diode (OLED). It comprises an anode, a cathode, and a layer including an organic compound or an inorganic compound (hereinafter referred to as an electro luminescent layer). The electro luminescent layer is interposed between the anode and the cathode and generates luminescence when an electric field is applied thereto. There is a relation between the amount of current flowing in the light emitting element and the luminance thereof, and the light emitting element emits light at a luminance corresponding to the amount of current flowing in its electro luminescent layer.
As a method for inputting a signal to a pixel when displaying a multi-gradation image in a light emitting display device, either an analog gray scale method (analog driving method) or a digital gray scale method (digital driving method) may be adopted. These methods are different in the way of controlling a light emitting element during a light emission or a non-light emission of the light emitting element.
In the analog gray scale method, gray scale is achieved by continuously controlling the amount of current flowing in a light emitting element. Meanwhile, in the digital gray scale method, the display device is driven by switching two states of the light emitting element, namely, an ON state (the luminance is substantially 100%) and an OFF state (the luminance is substantially 0%). In the digital gray scale method, however, not more than two-level gray scale display can be achieved without any complement. Therefore, an area gray scale method or a time gray scale method is suggested to be used as a driving method for displaying a multi-gradation image. In the time grayscale method, one frame period is divided into a plurality of subframe periods so as to have different lengths of light emitting time, and gray scale display is achieved by selecting the subframe periods. On the other hand, in the area gray scale method, a pixel is divided into subpixels so as to have different light emitting areas and gray scale display is achieved by selecting the subpixels.
A driving method when displaying a multi-gradation image in a light emitting display device is broadly classified into a voltage input method and a current input method. These two methods can adopt both the analog gray scale method and the digital gray scale method.
In the voltage input method, a video signal having a voltage value is input to a pixel and then to a gate electrode of a driving element in the pixel, whereby controlling the luminance of a light emitting element by the use of the driving element. When adopting the analog gray scale method in this case, the effect of variations in thin film transistors (TFTs) is increased.
In the current input method, a predetermined signal current is supplied to a driving element in a pixel and then to a light emitting element, whereby controlling the luminance of the light emitting element. In this case, multi-level gray scale is achieved in accordance with a current value supplied to the light emitting element. Accordingly, the amount of writing time is increased because of a signal input with current.
The luminance of a light emitting element in the aforementioned display device degrades with time (hereinafter referred to as the degradation with time) as shown below. FIG. 15A is a graph showing a voltage applied to a light emitting element relative to a current supplied to the light emitting element. The graph of FIG. 15A shows that when applying a voltage V0, a predetermined luminance is obtained at a current Ia firstly, but the luminance is lowered because the current drops to Ib even when applying the voltage V0 due to the degradation with time.
As shown in a graph of FIG. 15B which shows current-luminance characteristics of a light emitting element, the luminance of the light emitting element at a current I0 drops from La to Lb as time passes.
A light emitting element generates heat when voltage or current is applied thereto. Accordingly, film quality of an electro luminescent layer degrades or interfaces between the electro luminescent layers and electrodes degrade, and this is considered to be a cause of the luminance decay. Further, each light emitting element degrades differently, leading to image persistence.
Thus, in the case of adopting the voltage input method, suggested is a display device in which a uniform screen without variations in luminance can be achieved by correcting an image signal (video signal) depending on the correction data which is stored in advance in accordance with a degradation level of a self-light emitting element (see Patent Document 1). More specifically, it is a display device using the time gray scale method in order to display a multi-gradation image, and in the display device, one bit of subframe is used for correcting a video signal. For example, in the case of a light emitting display device using 6-bit digital gray scale, one bit of processing power is added for video signal correction, and the display device is designed and fabricated as the one for 7-bit digital gray scale. In a normal operation, the lower six bits are employed. Meanwhile, in the case where a light emitting element degrades, a correction value is added to a normal digital image signal and processed by using an additional one bit.
As shown in FIG. 18, the voltage input method may be combined with the current input method. In that case, each pixel of a light emitting display device is provided with a plurality of transistors PchTFTs 1 to 3 each gate electrode of which is connected with each other and which have different current capacities. The current supply capacity of each of the transistors is set to be proportional to an arbitrary current value. Then, whether to supply current to light emitting elements or not is controlled by turning ON/OFF switches SW1A to SW3A provided between drain electrodes of transistors and the light emitting elements in accordance with digital gray scale data D0 to D2. According to this, a high accuracy current which is affected only by variations in characteristics of transistors in the adjacent areas can be supplied to the light emitting elements (see Patent Document 2).
[Patent Document 1]
Japanese Patent Laid-Open No. 2002-175041
[Patent Document 2]
Japanese Patent Laid-Open No. 2003-66909